WONT STAY RUNNING 95 START

Okey here we go I have got this car buick 1995 riviera car wil start just fine. But wont stay running. I can get the car to stay runnig if I pump the gas and work at it. It will rive up [let off dies].I know check the gas pressue its [50] on gauge I dont think its the gas here. Why I say this. I just bought this thing for a good price or I hope?I guy said he had to change the key/lock in cloumn and he dont have the key flashed or car. He said thats when it started to do this. And it run just fine before the key/lock broke he put in new motor pick ups [timeing] I called a lock smith and they say thats what I need' [flashed] but I called some one that works on car's they said the car wouldnt start at all ? The reason I got this was for the motor. I have a 96 buick thats the same car [no tubo] I wanted to put the motor/ train's in it. But this thing is in better shape then my 96 if I can fix this and use the other for parts OBOY Iam to the piont were iam going to take it to someone [FIX IT]

I can check all that.I have new one's on my 96 riviera Are you sure it's not the int theft or key need's to be flashed?Why Im asking is I cant get the new code reader to read any code's at all just wont read the car I thought n maybe there was something wrong with my old code reader but it does the samething [no read]

This is what the car doing. Turn key fire's right up and die's. And it start's quick' if I doint put my foot on the gas and pump it forget it but it's real hard to keep it running i'v know I said this before' this is the same thing it's been doing sence I bought this thing. Are thinking maybe Iam loseing power from the key? If so I dont know what to think about that. I seem to have power [if I can pump the car & she run's ] I think we are heading in the right place [gas] I dont have the tools to check the fuel injector's. So iam going to have some one look at it [Shop] but I will let you know what I find out.